filtic

% Set initial condition vector for filter function
% The vector zf has the same values that would be obtained 
% from function filter given past inputs x and outputs y
%
% The vectors x and y contain the most recent inputs and outputs
% respectively, with the newest values first:
%
% x = [x(-1) x(-2) ... x(-nb)], nb = length(b)-1
% y = [y(-1) y(-2) ... y(-na)], na = length(a)-a
%
% If length(x)<nb then it is zero padded
% If length(y)<na then it is zero padded
%
% zf = filtic(b, a, y)
%    Initial conditions for filter with coefficients a and b
%    and output vector y, assuming input vector x is zero
%
% zf = filtic(b, a, y, x)
%    Initial conditions for filter with coefficients a and b
%    input vector x and output vector y
%